For me, the main reason is that it most closely matches the standard GTD workflow, i.e. basic lists of:
- Next Actions (single-step tasks)
- Projects (multi-step tasks)
- Waiting for (tasks that need external response)
- Someday (tasks that are nice to do, not essential)

I just could not find a good-enough reason to use folders for areas of focus. I tried but it didn't work for me. For example, if I have to do the following tasks:
(a) Complete & send a form to the insurance company
(b) Publish end-of-month financials
(c) Order software licence upgrades for SQL server

And how will I later find these tasks? E.g. task (a) might belong in insurance or admin, task (b) could be filed in finance or reporting, task (c) might belong in purchasing or software, etc.

And what value does it add to my system to have these tasks filed under their arbitrary 'area of focus' folder? What benefit do I derive from this? It's neat but what does it do for me?

Adding the status field to my workflow would just complicate things even further for me. I don't want to have to maintain each task to set its status to 'active' or 'complete' or whatever. I just want to action tasks and have them disappear off my list.

Priority is another field I turned off. Again, it's just arbitrary and would possibly change from day to day as other demands on my time arise, so why bother using it?

In the same way, GTD argues against setting due dates on tasks unless there really is a hard deadline. Using due dates to set priority is false economy, resulting in wasted time every day rolling them forward.

First and foremost, my GTD system needs to help me get things done efficiently, not slow me down. It's not meant to be a complicated database. It should be simple and effective.